As is well known, in a vehicle of a front-engine rear-wheel-drive (FR) type, torque generated by an engine and/or a motor is transmitted through a transmission and a propeller shaft to a final drive. The final drive contains a differential gear set and thereby distributes the torque to the right and left driving wheels.
The final drive may further contain a clutch for limiting differential motion of the differential gear set and an electric motor is often used to operate this. As an ordinary electric motor is somewhat powerless to drive the clutch, the motor is usually used in combination with reduction gears. The patent literatures indicated below are relevant background.
Japanese Patent Application Laid-open No. H02-173424 (“PTL1”).
Japanese Patent Application Laid-open No. 2002-262511 (“PTL2”).
Where to place the electric motor in the final drive is an important factor in durability. As the final drive is mounted under the vehicle's body, as being opposed to the road, it is steadily exposed to incoming stones, dirt or water. The electric motor is vulnerable to bad influences thereby.
As the art disclosed in the PTL 1 allows arrangement of the electric motor and the reduction gears above the casing, these elements are unlikely to suffer bad influences by the incoming stones, dirt and water. In this case, however, as the lubricant oil within the casing cannot sufficiently reach the reduction gears, an independent lubrication system separate from the one within the casing is required. Specifically, the art disclosed in PTL 1 requires that the reduction gears should be incorporated in a casing separated from the final drive and a lubricant such as grease should be filled therein. Further, particular attention should be necessary to adjust a shaft in position because the shaft drawn out of the casing for the reduction gears and intruding into the casing for the final drive bears transmission of the driving force. Fixation between these casings may give rise to any issue and such an arrangement may raise another issue that dusts intrude therein through a coupling therebetween. Its durability is not always sufficient.
In a case where the electric motor is disposed below the casing as with the art disclosed in the PTL 2, the lubricant oil within the casing can be supplied to the reduction gears. However, of course, the electric motor is then exposed to the incoming stones, dust and water from the road.